Considerable attention has been directed to the development of zinc electroplating baths which will produce zinc deposits of improved quality. Research has been devoted to improving the over-all brightness, the range of allowable current densities, and the ductility of the zinc coatings. Many of the successful aqueous alkaline zinc plating baths contain various quantities of sodium cyanide and various brightener compositions to improve the brightness of the zinc deposits. Although the alkaline cyanide baths generally are less expensive to operate and are particularly effective in plating substrates having recesses (such as cup shapes), the trend has been to develop zinc plating baths requiring lesser amounts of cyanide or no cyanide since the cyanides are toxic. The toxicity of the cyanide materials presents serious problems of disposal, particularly since waste disposal has come under the scrutiny of the federal and state governments resulting in the passage of a number of laws and regulations regarding the disposal of waste materials.
Alkaline zinc plating baths generally are based on a solution of zinc ions and an excess of a base such as sodium hydroxide and water. High pH alkaline zinc baths when used without brightening or addition agents yield deposits which are rough and spongy, and generally unacceptable for most applications. In a number of cases, production experience has demonstrated that alkaline zinc electroplating baths perform better if a small amount, e.g., less than 15 g/l, of cyanide is included in the bath.
Cyanide-free or substantially cyanide-free alkaline zinc plating baths containing various organic additives have been described in the prior art. U.S. Pat. No. 3,849,325 relates to the deposition of a bright zinc coating from an alkaline zinc electroplating bath which contains little or no cyanide. The brightening agent which is incorporated into the bath is obtained by the reaction of a nitrogen-containing heterocyclic compound such as pyridine, pyrazine, quinoline and derivatives thereof, an acyclic amine having at least two functional groups, formaldehyde and an epihalohydrin or a glycerol halohydrin.
Organic additives also have been developed which, when included in the cyanide-free plating baths, do provide electrodeposits of fair quality. Efforts have continued in the art to develop brightener additive compositions which will significantly improve the brightness and ductility of the zinc deposit. In U.S. Pat. No. 3,974,045, a water-soluble polymer additive for alkaline zinc plating baths is described which is prepared by reacting at least one epihalohydrin with at least one nitrogen heterocyclic compound such as the compounds of imidazole, pyrrole, cyclic amines and piperazine in the presence of at least one nitrogen compound such as the compounds of aliphatic amines, hexamethylene tetramine and ammonia. The brightener additive is useful in both cyanide-containing and cyanide-free alkaline zinc plating baths.
U.S. Pat. No. 4,397,717 describes alkaline zinc plating baths containing one or more reaction products obtained by first reacting an epihalohydrin with a heterocyclic nitrogen compound having at least two nitrogen atoms to form an intermediate, and then reacting the intermediate with ammonia, an aliphatic amine, polyamine and/or polyimine. The bath also contains the reaction product of a polyvalent alcohol with epichlorohydrin and a nitrogen-containing heterocyclic compound.
U.S. Pat. No. 4,188,271 describes compositions prepared by reacting formaldehyde with a mixture of one or more piperazines and one additional nitrogen compound such as ammonia or aliphatic, alicyclic compounds containing at least one primary amine group to form an intermediate which is then reacted with an epihalohydrin or glycerol halohydrin. These compositions are intended for use in aqueous alkaline zinc electroplating baths for producing bright metallic zinc deposits.
U.S. Pat. No. 4,169,772 describes acidic zinc plating baths containing a brightener composition obtained by reacting a nitrogen-containing heterocyclic compound with formaldehyde, an epihalohydrin or glycerol halohydrin and at least one amino compound which may be ammonia, an aliphatic amine or an amidine.